Bypass engines using a ducted thrust fan driven by a turbine power plant are commonly utilized as propulsion units on aircraft. The ducted turbofan unit of such an engine comprises a thrust fan housed within a surrounding by-pass duct disposed thereabout. The fan is typically mounted to the forward end of the turbine power plant in front of the intake to the compressor of the turbine power plant, although rear mounted fans are also known. In any case, the fan is rotatably driven by the turbine power plant, either separately from or jointly with the compressor.
The fan includes a plurality of rotor blades extending radially outward from the hub at equally spaced intervals about the circumference of the hub. The particular number of blades provided and the pitch character of the blades, i.e. whether fixed-pitch or variable-pitch, depends upon the particular application, as well as size of the engine. The by-pass duct is disposed coaxially about the hub of the fan and the engine axis in closely spaced relationship to the outboard tips of the fan blades. It is well appreciated in the art that minimal clearances between the outboard tips of the fan blades and the inner wall of the surrounding duct must be maintained in order to assure the high aerodynamic fan efficiency critical to providing maximum thrust output and fuel economy. With minimal clearances, it is inevitable that during operation the tips of the blades will occasionally contact the surrounding fan duct due to centrifugal forces, thermal effects or vibrations.
Accordingly, it is customary practice to provide a replaceable circumferential wear strip of abradable material in the inner wall of the fan duct in the region thereof surrounding the fan blade tips, the wear strip material having a relatively low wear resistance as compared to the fan blades. Traditionally, fan blades for use in ducted fans have been made of titanium alloys which have a relatively high wear resistance compared to the abradable strip. Thus, when the tips of the titanium blades do occasionally contact the abradable strip as the blades rotate in the fan duct, the tips of the titanium blades wear a track in the sacrificial layer of abradable material thereby permitting the maintenance of minimal clearance between the blade tips and the surrounding fan duct.
Although titanium alloy fan blades have proven very effective in ducted fan applications, the continuing need for improved fuel economy has spurred the search for lower weight fan blades. For example, commonly assigned U.S. Pat. No. 3,799,70 discloses a composite fan blade for use in ducted fan aircraft turbine engines formed by bonding complementary preformed outer shell halves defining the blade planform about a central load carrying spar which extends outwardly beyond the root end of the planform for attaching the composite blade to the fan shaft. The blade planform is fabricated from a composite material having a high tensile modulus such as boron epoxy, boron-aluminum or carbon, while the spar is made from titanium.
Such composite blades can successfully be used in ducted fan applications so long as the sacrificial layer in the surrounding region of the fan duct is made of a material which has a substantially lower wear resistance. For maintenance of maximum fan efficiency, it is highly desirable to minimize wear of the blade tips and permit wear of the sacrificial abradable layer. Heretofore, composite blades made of fiber reinforced epoxy resins, although being of very low weight as desired, have been less suitable for use in ducted fans because of the extremely low wear resistance of the epoxy resin material relative to the materials customarily used in abradable layer of the fan duct.
Accordingly, it is a general object of the present invention to provide a lightweight composite blade equipped with a tip having a relatively high wear resistance thereby rendering the blade suitable for use in ducted rotary devices, such as fans and compressors.